1. Field of the Invention
The present invention relates to a process for forging a titanium-based material. More particularly, it relates to a process for forging a titanium-based alloy, process which is used to make an automobile engine valve.
2. Description of the Related Art
The requirements for the materials of engine valves, which are installed to automobile combustion engines, are classified as the most severest ones in the engine component parts. In particular, the engine valves are subjected to considerably large loads while they are exposed to high-temperature combustion atmospheres. Accordingly, the engine valves are required to exhibit a heat resistance, a corrosion resistance, an oxidation resistance, and a wear resistance against the seating surfaces of the seats at elevated temperatures. Moreover, as the recent trend toward high-performance automobiles develops, the engine valves are required to be lightweighted.
As an engine valve satisfying these requirements, an engine valve is developed which uses a titanium-based material (or a titanium alloy).
In the titanium alloy, the characteristics are closely related to the crystal structures. Therefore, the titanium alloy is roughly divided into an xcex1-titanium alloy, an xcex1+xcex2-titanium alloy and a xcex2-titanium alloy according to the crystal structures.
It has been known that the xcex1+xcex2-titanium alloy, which is used in the largest amount, transforms to a xcex2 phase at a transformation temperature ( xcex2-transus temperature) or more ( xcex2 phase region), and that the titanium alloy having the xcex2 phase transforms to an xcex1+xcex2-structure at the xcex2-transus temperature or less (xcex1+xcex2 phase region).
The xcex1+xcex2-titanium alloy is rapidly turned into a coarse microstructure when the xcex2-transus temperature is exceeded, and exhibits a decreased impact value and a reduced fatigue strength. Accordingly, the forging of the conventional xcex1+xcex2-titanium alloy is carried out in the xcex1+xcex2 phase region. However, since the xcex1+xcex2-titanium alloy exhibits a large resistance to deformation in the xcex1+xcex2 phase region, it is difficult to carry out the forging.
The titanium alloy engine valve, which is processed out of such a titanium alloy, is generally manufactured in the following manner. A titanium alloy rod material is manufactured from an ingot titanium alloy, and is molded preliminarily by an upsetter. The upset portion is hot swaged so as to form a valve shape.
For example, Japanese Unexamined Patent Publication (KOKAI) No. 7-34,815 discloses a process for producing a titanium alloy engine valve. In this production process, a titanium alloy rod is hot extruded, and is swaged with a mold to an umbrella-like shape at the end.
Another a process is for manufacturing an engine valve by the powder metallurgy method. Namely, a titanium alloy powder is compacted to a molded substance having a valve shape by the cold isostatic pressing (CIP), and thereafter the compact having a valve shape is sintered.
As an example of such a powder metallurgy method, a process for producing an engine valve is disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 6-229,213. In the publication, there is disclosed the following process for producing an engine valve. Namely, a mixture of a titanium powder and an aluminum powder is subjected to the canning so that it is extruded and forged into a valve shape, and is thereafter reacted to synthesize Tixe2x80x94Al intermetallic compounds, thereby producing an engine valve comprising the Tixe2x80x94Al intermetallic compounds.
However, in the process for producing an engine valve set forth in Japanese Unexamined Patent Publication (KOKAI) No. 7-34,815, the titanium alloy rod material is used. Since the titanium alloy rod material is a cast material, it is necessary to provide a large number of processes for manufacturing the rod material and for turning it into a straight rod shape. In addition, since the material yield is bad, and accordingly the cost goes up.
In the production process for producing an engine valve set forth in Japanese Unexamined Patent Publication (KOKAI) No. 6-229,213, the powder metallurgy method is used. Since the as-sintered body has many residual pores, the resulting engine valve has a problem in that it exhibits the low ductility and fatigue strength.
The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to provide a process for forging a titanium-based material, process which can produce titanium-based material products of high ductility and fatigue strength at a low cost, and to provide a process for producing an engine valve.
In order to achieve the aforementioned object, the inventors of the present invention investigated into the processes for producing titanium-based materials. As a result, it was possible to carry out forging under a temperature condition where a material exhibited less resistance to deformation and to keep a fine alloy structure by hot forging a titanium-based sintered workpiece which included ceramics, which were thermodynamically stable in a titanium alloy, or pores. Accordingly, it was confirmed that the impact value and the fatigue strength were inhibited from decreasing. Thus, the inventors discovered that the aforementioned problems could be overcome.
Namely, a process for forging a titanium-based material according to the present invention is characterized in that it comprises the steps of:
preparing a titanium-based sintered workpiece including at least one of ceramics particles and pores in a total amount of 1% or more by volume, the ceramics particles being thermodynamically stable in a titanium alloy; and
heating the workpiece to a forging temperature and forging the same.
The ceramics particles which are thermodynamically stable in a titanium alloy can be titanium boride, titanium carbide, titanium silicide, and titanium nitride. The titanium boride can be TiB and TiB2. The titanium carbide can be TiC and Ti2C. The titanium nitride can be TiN. In a wider sense, the ceramics particles include intermetallic compounds and oxides of rare-earth elements as well. Among them, the titanium boride is preferred. The phrase, xe2x80x9cthermodynamically stable in a titanium alloyxe2x80x9d, means that the ceramics particles can exist as particles and reside in a titanium alloy without decomposing and solving therein up to elevated temperatures. It does not necessarily mean that the ceramics particles require a heat resistance strength. As far as the ceramics particles exist as particles, they operate and effect advantages similarly. The ceramics particles can preferably have an average particle diameter of from 1 to 40 xcexcm.
A process for producing an engine valve according to the present invention is characterized in that it comprises the steps of:
heating a sintered billet;
extruding the heated billet with a part thereof unextruded, thereby forming a stem;
rolling the extruded stem, thereby correcting an axial flexure thereof;
re-heating the sintered billet; and
hot upsetting the unextruded part, thereby forming a head.
When the titanium-based material is simply sintered, it suffers from the degradation in terms of the ductility and the fatigue strength by the residing pores. However, since compacting is carried out by forging, no degradation of the ductility and the fatigue strength occurs.
In the present titanium-based material production process, since the sintered body forged, the degradation of the ductility and the fatigue strength resulting from the residing pores can be suppressed. Thus, the present titanium-based material production process can produce forged products whose characteristics are equal to those of ingot metal.
Moreover, in the present engine valve production process, since the sintered billet is used, the processes up to the manufacturing of the billet are shortened remarkably.